The present invention concerns a method of reshaping a workpiece, in particular an upsetting riveting method, and a device for reshaping a workpiece.
In the reshaping operation, that is to say upon specifically targeted plastic deformation of a workpiece, a force is applied to the workpiece by way of a reshaping tool and the workpiece is thereby reshaped. A distinction is drawn between pressure reshaping, tension-pressure reshaping, tension reshaping, bending reshaping and thrust reshaping.
Particularly in the pressure reshaping process the reshaping tool has an upsetting surface of a contour which substantially corresponds to the desired outside contour of the workpiece to be reshaped. During the reshaping operation that upsetting surface is brought into contact with the workpiece to be reshaped and a force is applied to the workpiece so that it is thereby reshaped.
In the case of many materials it is advantageous if the workpiece to be reshaped is heated during or immediately prior to the reshaping operation.
For heating the workpiece it is possible for example for an ultrasonic vibration, that is to say a vibration between about 16 kHz and about 10 GHz, to be applied to the workpiece. That ultrasonic vibration is absorbed in the workpiece and the workpiece is heated thereby.
Thus it is known for example for a rivet to be reshaped by means of a closing head shaper in the form of a sonotrode. FIGS. 1a through 1c diagrammatically show the method steps applied in a known reshaping method.
The known riveting method is used to join two materials or two elements together. Firstly the rivet 1 comprising a plastic is passed through an opening arranged in the riveting material 2. The rivet 1 is either connected to one of the two elements to be joined or it has a prefabricated setting head which prevents the rivet from being able to be pressed completely through the opening in the riveting material 2.
As can be seen from FIG. 1 in that situation the sonotrode 3, that is to say an element which is acted upon with an ultrasonic vibration, is moved in the direction of the riveting material 2 so that firstly the edges of the end face of the rivet 1 come into contact with the sonotrode 3. The contact of the sonotrode 3 with the rivet 1 means that an ultrasonic vibration is transmitted into the rivet. The ultrasonic vibration is absorbed in the material. The proportion of the absorbed ultrasonic energy depends on the damping constant or the absorption coefficient of the material.
In general the amplitude of the ultrasonic vibration will decrease in the workpiece so that the greatest amplitude is reached at the contact surface relative to the sonotrode and becomes progressively smaller, the further the ultrasonic wave moves away from the sonotrode.
The absorption effect results in heating of the rivet 1. As in the known method the rivet 1 comprises a plastic the material will begin to melt, as shown in FIG. 1b and finally, as shown in FIG. 1c, the material is reshaped by the closing head. Basically, it is possible by means of ultrasound for the workpieces to be shaped to be heated very quickly and in particular only locally so that the desired hot reshaping can take place.
With some materials however the introduction of heat by the ultrasonic vibration is limited to a close region in the proximity of the contact surface relative to the sonotrode That is due on the one hand to the absorption which generally falls exponentially with the spacing relative to the sonotrode. On the other hand those materials exhibit only a low level of thermal conductivity so that the heating effect firstly remains restricted substantially to a portion in the immediate proximity of the sonotrode. If now in addition the workpiece to be reshaped comprises a material which has a heavily temperature-dependent absorption coefficient such that the absorption coefficient becomes greater with temperature, absorption in the region directly in the proximity of the sonotrode is still further increased by the local heating effect, which results in even more greatly localized heating.
In the known method therefore it is only possible for a region in the immediate proximity relative to the sonotrode to be adequately heated, so that it is also only in those regions that effective reshaping take place.
As can be seen in particular from FIG. 1c the closing head 5 produced in that way has cylindrical constriction recesses 4 and very large regions which are actually not homogeneously connected to the rivet, but are only placed around the cylinder. Those regions reduce the stability of the closing head 5. In practice therefore the closing heads produced in that way have to be of larger dimensions than would actually be necessary in consideration of their geometry. Nonetheless even then this does not guarantee adequate strength.
Therefore, based on the described state of the art, the object of the present invention is to provide a method of and a device for reshaping a workpiece, which particularly when reshaping materials with a heavily temperature-dependent damping constant, avoids the above-mentioned disadvantages.